How to downgrade from Denim to Cyan

Hello,
I was waiting eagerly for the Denim update for several months, just to find out that this update is an utter failure in terms of battery life.

the reduction is between 30 and 50% less battery life.

Thank you, but no, I don't need cortana and faster lumia camera, if it is at the cost of double battery drain speed.
I want to return to Cyan, which was able to stay in standby for more than 3 weeks (Lumia 1520), and I was able to surf for 6 hours, or watch 10 hours of video, instead of half in Denim.

Tried soft reset (I guess the description for it - like to remove the battery is different than just normal restart when you power off and turn on again) - it actually showed one of the worst results - 14 % battery drain for 24 minutes of Moli player MKV video - task which was 3 to 5 % before the Denim upgrade....
The VCL when playing AVI is still capable of 10% for 1 hour, but it is nothing compared to the 6% before denim.
Will try the hard reset now during the orthodox eastern holidays... But not really expecting the issue to be solved.